Empirical H alpha emitter count predictions for dark energy surveys

Future galaxy redshift surveys aim to measure cosmological quantities from the galaxy power spectrum. A prime example is the detection of baryonic acoustic oscillations, providing a standard ruler to measure the dark energy equation of state, w(z), to high precision. The strongest practical limitation for these experiments is how quickly accurate redshifts can be measured for sufficient galaxies to map the large-scale structure. A promising strategy is to target emission-line (i.e. star-forming) galaxies at high redshift (z similar to 0.5-2); not only is the space density of this population increasing out to z similar to 2, but also emission lines provide an efficient method of redshift determination. Motivated by the prospect of future dark energy surveys targeting Ha emitters at near-infrared wavelengths (i.e. z > 0.5), we use the latest empirical data to model the evolution of the Ha luminosity function out to z similar to 2 and thus provide predictions for the abundance of Ha emitters for practical limiting fluxes. We caution that the estimates presented in this work must be tempered by an efficiency factor, epsilon, giving the redshift success rate from these potential targets. For a range of practical efficiencies and limiting fluxes, we provide an estimate of (n) over barP(0.2), where (n) over bar is the 3D galaxy number density and P-0.2 is the galaxy power spectrum evaluated at k = 0.2 h Mpc (1). Ideal surveys must provide (n) over barP(0.2) > 1 in order to balance shot-noise and cosmic variance errors. We show that a realistic emission-line survey (epsilon = 0.5) could achieve (n) over barP(0.2) = 1 out to z similar to 1.5 with a limiting flux of 10 (16) erg s (1) cm (2). If the limiting flux is a factor of 5 brighter, then this goal can only be achieved out to z similar to 0.5, highlighting the importance of survey depth and efficiency in cosmological redshift surveys.